1. Field of the Invention
The present invention relates to a gear shift control system of an automatic transmission installed in an automobile, and, more particularly, to an automatic transmission gear shift control system in which at the same time that a first frictional element among a plurality of frictional elements is coupled, a second frictional element is released or unlocked and to a method for same.
2. Description of Related Art
Typically, an automatic transmission installed in an automobile combines a torque converter and a transmission-gear mechanism, and is provided with a plurality of frictional elements, such as brakes and clutches, which switch the torque transmission paths of the transmission-gear mechanism, as well as a hydraulic circuit which controls the application of hydraulic pressure for engagement to these frictional elements and the discharge of hydraulic pressure from them. Such an automatic transmission is designed to shift automatically gears according to driving conditions by conducting selective application of hydraulic pressure for engagement to the plurality of frictional elements which is performed by pre-designed operation of actuators in the hydraulic circuit. In this hydraulic circuit there are provided various valves, such as a regulator valve which regulates a pressure discharged from an oil pump to a specified line pressure, a manual shift valve which is manually operated to shift gear ranges, and a plurality of shift valves which are automatically operated according to vehicle driving conditions so as to cause the actuators to selectively lock and unlock the frictional elements.
With regard to this type of automatic transmission, some frictional elements are simultaneously locked and/or unlocked during a specific gear shift. For example, with an automatic transmission of this type which can be selectively shifted to first (1st) to fourth (4th) gears and is provided an frictional element which is locked for third (3rd) and fourth (4th) gears and a frictional element which is locked for second (2nd) and fourth (4th) gears, at the time that the former frictional element is locked, the later frictional element is unlocked during, for example, an up-shifting of second (2nd)-to-third (3rd) gear.
In such an automatic transmission where a plurality of frictional elements are selectively locked and unlocked at the same time in order to execute shifting of gear, a timely management of locking and unlocking of the frictional elements is quite important in order to complete smoothly shifting of gear. During the up-shifting of second (2nd)-to-third (3rd) gear, if the execution of unlocking of the frictional element for second (2nd) and fourth (4th) gears is made comparatively earlier than the execution of locking of the frictional element for third (3rd) and fourth (4th) gears, the transmission mechanism transitionally shifts through a neutral range, resulting in an occurrence of a racing of engine. In contrast, in the event where the execution of unlocking of the frictional element for second (2nd) and fourth (4th) gears is made comparatively later than the execution of locking of the frictional element for third (3rd) and fourth (4th) gears, the transmission mechanism is temporarily brought into what is called a "double-locked" state, so as to cause a shift shock due to a drop of output torque.
In order to eliminate such drawbacks of the automatic transmission of this type, the timing of locking and unlocking of the frictional elements must be properly regulated.
Essentially, the regulation of a timing at which locking and unlocking of the frictional elements are made is performed by means of passing hydraulic pressure through orifices which are formed in pressure lines leading to actuators of the respective frictional elements. Furthermore, for more precisely regulating the timing, each pressure line is accompanied by a bypass pressure line which is opened and closed by a timing valve so as to allow and prohibit the hydraulic pressure to bypass the orifice. This timing valve is operated during a shifting of gear so as to conduct the timing of supply and discharge of hydraulic pressure through the bypass line quickly for a first half of gear shift operation and through the orifice generously for a second half of gear shift operation, thereby precisely controlling the timing. Such an automatic transmission control means is known from, for instance, Japanese Unexamined Patent Publication No. 2-76968.
However, even in the event where a timing valve is installed to control the timing of supply and discharge of hydraulic pressure, the automatic transmission is subjected to a racing of engine and a shift shock. That is, with the configuration in which a hydraulic circuit is provided, in a pressure discharge line leading to the frictional element for second (2nd) and fourth (4th) gears which are unlocked during 2-3 shift-up, with an orifice which stops down the pressure discharge line, a bypass pressure line bypassing the orifice, and a timing valve installed in the bypass pressure line which opens the bypass pressure line for a predetermined period for a first half of gear shift operation, even though the period for which the timing valve opens the pressure bypass line is established to be unchanged, the level of pressure is not always stable while the pressure bypass line is closed. This is because, as the viscosity of working oil changes according to its temperature, the working oil in the automatic transmission is subjected to deterioration of fluidity. Accordingly, if the temperature of working oil is lowered in excess, unlocking of the frictional element for the second (2nd) and fourth (4th) gears is advanced in excess ahead locking of the frictional element for the third (3rd) and Fourth (4th) gears, resulting in a racing of engine. On the other hand, if the temperature of working oil rises in excess, unlocking of the frictional element for the second (2nd) and fourth (4th) gears is delayed in excess, resulting in a shift shock due to double-lock.